Rename PackageName.main to .base to making naming consistent

[s-ludwig]

This is and has always been referred to "base package name" throughout the code base and should be named like this here, too, to avoid confusion.

Also reduces the amount of explicit attribute annotations a bit.

[github-actions]

✅ PR OK, no changes in deprecations or warnings

Total deprecations: 0

Total warnings: 0

Build statistics:

 statistics (-before, +after)
 executable size=5265688 bin/dub
 rough build time=56s

Full build output

DUB version 1.40.0, built on Jun  7 2025
LDC - the LLVM D compiler (1.41.0):
  based on DMD v2.111.0 and LLVM 20.1.5
  built with LDC - the LLVM D compiler (1.41.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: icelake-server
  http://dlang.org - http://wiki.dlang.org/LDC


  Registered Targets:
    aarch64     - AArch64 (little endian)
    aarch64_32  - AArch64 (little endian ILP32)
    aarch64_be  - AArch64 (big endian)
    amdgcn      - AMD GCN GPUs
    arm         - ARM
    arm64       - ARM64 (little endian)
    arm64_32    - ARM64 (little endian ILP32)
    armeb       - ARM (big endian)
    avr         - Atmel AVR Microcontroller
    bpf         - BPF (host endian)
    bpfeb       - BPF (big endian)
    bpfel       - BPF (little endian)
    hexagon     - Hexagon
    lanai       - Lanai
    loongarch32 - 32-bit LoongArch
    loongarch64 - 64-bit LoongArch
    mips        - MIPS (32-bit big endian)
    mips64      - MIPS (64-bit big endian)
    mips64el    - MIPS (64-bit little endian)
    mipsel      - MIPS (32-bit little endian)
    msp430      - MSP430 [experimental]
    nvptx       - NVIDIA PTX 32-bit
    nvptx64     - NVIDIA PTX 64-bit
    ppc32       - PowerPC 32
    ppc32le     - PowerPC 32 LE
    ppc64       - PowerPC 64
    ppc64le     - PowerPC 64 LE
    r600        - AMD GPUs HD2XXX-HD6XXX
    riscv32     - 32-bit RISC-V
    riscv64     - 64-bit RISC-V
    sparc       - Sparc
    sparcel     - Sparc LE
    sparcv9     - Sparc V9
    spirv       - SPIR-V Logical
    spirv32     - SPIR-V 32-bit
    spirv64     - SPIR-V 64-bit
    systemz     - SystemZ
    thumb       - Thumb
    thumbeb     - Thumb (big endian)
    ve          - VE
    wasm32      - WebAssembly 32-bit
    wasm64      - WebAssembly 64-bit
    x86         - 32-bit X86: Pentium-Pro and above
    x86-64      - 64-bit X86: EM64T and AMD64
    xcore       - XCore
    xtensa      - Xtensa 32
   Upgrading project in /home/runner/work/dub/dub/
    Starting Performing "release" build using /opt/hostedtoolcache/dc/ldc2-1.41.0/x64/ldc2-1.41.0-linux-x86_64/bin/ldc2 for x86_64.
    Building dub 1.41.0+commit.8.g52ac2d8b: building configuration [application]
     Linking dub
STAT:statistics (-before, +after)
STAT:executable size=5265688 bin/dub
STAT:rough build time=56s

[the-horo]

Some tests failure because setup-dlang@v2 changed the way DC is set:

[INFO] Running /home/runner/work/dub/dub/test/dc-env.sh...
core.exception.AssertError@app.d(17): Expected 'dmd' but got '/opt/hostedtoolcache/dmd/2.112.0/x64/dmd2/linux/bin64/dmd'
----------------
??:? _d_assert_msg [0x55936729e140]
./app.d:17 _Dmain [0x55936727c875]
Error Program exited with code 1
Error:  /opt/hostedtoolcache/dmd/2.112.0/x64/dmd2/linux/bin64/dub:6 command failed
[ERROR] Script failure.

I guess I should merge dlang-community/setup-dlang#94 since it's been staying there all this time, then you could do:

- name: Install latest DMD
   uses: dlang-community/setup-dlang@v2
   with:
     dc_format: basename

and the test should pass. Alternatively, making it less flasky is also a good idea: https://github.com/the-horo/dub/blob/ba66cf37fd8597fa830805872d169be1a957150d/test/new_tests/issue2012-dc-env/sample/app.d